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2013WinShotcreteEMag

Shotcrete FAQs surface are the shotcrete rod, wood Please note: ASA’s technical team provides the answers to submitted questions as a floats, and steel trowels. Typically, shot- free service. The information is based on the personal knowledge and experience of crete walls are a minimum of 4000 psi the ASA technical team and does not represent the official position of ASA. We assume (27.6 MPa) 28-day compressive strength that the requester has the skills and experience necessary to determine whether the and the aggregate varies from sand only information ASA provided is appropriate for the requester’s purposes. The information to a blend of sand and 0.375 to 0.5 in. provided by ASA is used or implemented by the readers at their OWN RISK. (9.6 to 13 mm) aggregate. Consult our Shotcrete magazine archives for exam- ples at www.shotcrete.org. Question: In regards to the strength of shotcrete and weather is there a Mine Safety and Health Administration (MSHA) regulation requiring our paste cylinder sample results to be above a certain psi (MPa)? Answer: Shotcrete is normally expected to meet or exceed 4000 psi (27.6 MPa). We are not aware of any specific MSHA requirements. We would suggest you consult a tunnel or mining engineer who is well-versed with shotcrete. Cores taken from field-shot test panels are generally used for the evaluation of compressive strength of shotcrete (ASTM C1140/ C1140M-11). Question: We plan to use fiber-rein- forced (polypropylene fibers) shotcrete as a brown coat for stucco (three-coat stucco) over a concrete shear wall. Does a maximum thickness of 1.5 in. (38 mm) of shotcrete require any mechanical anchor/connection, or is the bonding strength of the shotcrete layer to the concrete shear wall substrate sufficient? Answer: The addition of fiber will not increase the bond of shotcrete to the concrete shear wall. A 1.5 in. (38 mm) thick layer of properly designed and applied shotcrete should have adequate bond to a properly prepared concrete substrate without additional mechanical anchors. However, exposure conditions, geometry of the wall, shrinkage potential of the shotcrete mixture, application technique, and curing—as well as the age and quality of the shear wall concrete substrate—may affect the bond. These factors should be considered by an engineer experienced with shotcrete overlays in deciding whether additional anchoring is advisable. Shotcrete • Winter 2013 69


2013WinShotcreteEMag
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